This invention relates to a seal arrangement for a gas turbine engine and more particularly to a seal arrangement for an inter-shaft seal that seals between coaxial rotor shafts in a multiple spool gas turbine engine.
A front bearing compartment in a multiple spool gas turbine engine is filled with an oil mist to lubricate bearings that support coaxial low pressure and high pressure rotor shafts. The low pressure rotor shaft is driven by a low pressure turbine exposed to hot exhaust gases from the combustor to drive a low pressure compressor and/or propeller. The high pressure rotor shaft is driven by a high pressure turbine upstream of the low pressure turbine and immediately adjacent to the combustor, and serves to drive the high pressure stage compressor immediately upstream of the combustor. The coaxial low and high pressure rotor shafts are separated by a gap filled with working medium gas. The working medium gas is warmer than the temperature inside the bearing compartment. An inter-shaft seal controls the amount of working medium gas leaking into the oil compartment and prevents the oil mist from leaking out of the compartment into the gap.
Flying birds are a danger to aircraft, particularly when they collide with the aircraft and are ingested into the propeller and compressor area of the gas turbine engine. During a bird strike the low pressure rotor shaft can be deflected beyond its operating limits. In order to compensate for the excess deflection of the low pressure rotor shaft and thereby avoid the resulting inter-shaft rubbing, the gap between the coaxial low and high pressure rotor shafts increases accordingly. Efforts have been made to improve bearings which have a combination of properties to allow normal running of the engine as well as allowing safe engine operation during unplanned rotor imbalances, such as those occurring as a result of bird strikes. Examples of such bearings are described in U.S. Pat. No. 5,433,584, issued to Amin et al. on Jul. 18, 1995, and U.S. Pat. No. 4,375,906, issued to Roberts et al. on Mar. 8, 1983.
During a bird strike event, the sealing of the bearing compartments at the shaft interfaces cannot be maintained unless another bearing or a bumper is added at the location of the sealing, to minimize the deflection. In order to solve this problem, U.S. Pat. No. 6,196,790, issued to Sheridan et al. on Mar. 6, 2001, for example, describes a complex seal assembly for an inter-shaft seal in a gas turbine engine. The seal assembly includes a first seal that seals to a seal plate on an inner rotor shaft, a second seal that seals to a seal plate on an outer rotor shaft, and an intermediate seal that is supported by a stator assembly and seals to the first seal and the second seal. The seal assembly accommodates relative radial and axial displacement between the inner and outer shafts. However, in order to provide this freedom of displacement, the mechanism is relatively complex and would appear to be rather expensive to manufacture and maintain. The individual seals of this seal assembly in combination, provide the inter-shaft seal. The seal assembly however, will fail if any of the individual seals malfunctions.
U.S. Pat. No. 5,403,019, issued to Marshall on Apr. 4, 1995, as another example, describes a free floating labyrinth ring seal to seal against a rotatable shaft to control excessive leakage of gas between high and low pressure chambers such as in a turbo machine. The seal includes an elastomeric ring-shaped body with spaced opposing high and low pressure walls and a labyrinth seal surface extending therebetween for sealing with the shaft. The walls have portions defining opposing surfaces of unequal area for pressure-balancing against each other within the cavity with a controlled gas flow clearance between the seal body and the walls of the cavity so that the seal body is substantially free to float with vibration of the shaft while maintaining close tolerance sealing between the labyrinth surface and the shaft as the latter rotates. This seal permits relative axial movement as well as relative radial movement to a limited degree. The seal is designed to float within a groove of a housing. The undercutting of the outside shaft with the groove to accommodate the sliding seal would appear to somewhat undermine the structural integrity of the shaft, as well as introducing mechanical complexity if such a configuration is used for an inter-shaft seal.
Therefore, there is a need for a seal arrangement for inter-shaft sealing in a gas turbine engine which is simply configured and meets seal leakage requirements for normal engine operating conditions, while also ensuring engine operability when a rotor unbalancing event occurs.
One object of the present invention is to provide a seal arrangement between coaxial inner rotating and outer rotating shafts of a gas turbine engine to ensure a working condition during both normal engine operation and during abnormal engine operation when a rotor unbalancing event occurs.
It is another object of the present invention to provide a seal arrangement between coaxial inner rotating and outer rotating shafts of a gas turbine engine which includes main and backup seals to provide a double seal function.
It is a further object of the present invention to provide a simple configuration of a seal arrangement between coaxial inner rotating and outer rotating shafts of a gas turbine engine.
In accordance with one aspect of the present invention, a method of sealing a radial gap between coaxial inner rotating and outer rotating shafts of a gas turbine engine is provided. The method comprises steps of using a main seal to seal the gap and using a back-up seal for a double seal of the gap. The main seal meets leakage requirements for a normal engine operation condition, and the back-up seal meets leakage requirements which ensures an engine operable condition in abnormal events. In such abnormal events the coaxial relationship between the inner rotating and outer rotating shafts is affected by the events during the engine operation, thereby causing a failure of the main seal.
The back-up seal is preferably adapted to accommodate relative radial displacement between the inner rotating and outer rotating shafts. Nevertheless, leakage requirements for the back-up seal are less preferably restrictive than leakage requirements for the main seal. A radial seal between an outer surface of the inner rotating shaft and an inner surface of a outer rotating shaft, according to one embodiment of the present invention, is used to act as the main seal, and an axial seal between an annular radial surface of the inner rotating shaft and annular radial surface of the outer rotating shaft is used to act as the back-up seal.
In accordance with another aspect of the present invention, an axial and radial seal arrangement is provided for use in a gas turbine engine to seal a radial gap between coaxial inner rotating and outer rotating shafts. The seal arrangement comprises a radial seal and an axial seal. The radial seal is disposed between an outer surface of the inner rotating shaft and an inner surface of the outer rotating shaft, acting as a main seal for a normal operation of the gas turbine engine. The axial seal is disposed between an annular radial surface of the inner rotating shaft and an annular radial surface of the outer rotating shaft, acting as a back-up seal for an abnormal condition in which the radial seal fails.
It is preferable that the axial seal permits a radial displacement between the inner and outer rotating shafts while maintaining seal function when the relative radial displacement causes the failure of the radial seal. In one embodiment of the present invention, the axial seal is disposed between an annular radial end surface of the outer rotating shaft and the annular radial surface on a flange extending radially and outwardly from the outer surface of the inner rotating shaft. The flange is preferably integrated with the inner rotating shaft.
Testing for bird strikes has shown that when a medium bird strike event occurs the deflection of the low pressure rotating shaft caused by the unbalanced rotor is substantially radial and the relative axial displacement between the inner and outer rotating shafts is practically ignorable, in contrast to the relative radial displacement. Therefore, a complex seal assembly which can accommodate relative radial and axial movement simultaneously, is not necessary in order to ensure an operable condition of the gas turbine engine when a medium bird strike event occurs. The present invention advantageously provides a simple configuration of an axial and radial seal arrangement for a double seal of the gap between the coaxial low and high pressure rotor shafts. In the arrangement, the radial seal which is usually better than an axial seal in terms of sealing results, is used for normal engine operation. The radial seal nevertheless, is prone to failure when bird strike events cause rotor imbalance. During such an event, as a back-up seal, the axial seal is much more tolerant than the radial seal and is therefore capable of maintaining an operable seal condition of the gas turbine engine to permit the air craft to continue flight for safe landing.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment of the present invention described hereinafter.